1. Field of the Invention
The present invention relates to a power supply apparatus provided with a plurality of batteries.
2. Description of the Background Art
Conventionally, a so-called battery pack made up of a plurality of batteries has been used as a power supply apparatus which supplies electric power for driving a potable electrical apparatus such as a cellular phone, a digital video camera and a notebook personal computer. A plurality of batteries are also used in a large power-supply apparatus employed as a back-up power-supply apparatus at a factory, a hospital or the like, an automobile-motor power source or the like. A lithium-ion secondary battery has been widely used as a battery for such a power supply apparatus.
A lithium-ion secondary battery has a great electromotive force though it is light, and a high energy density. These characteristics are useful to a power source for driving a potable electronic apparatus of each type such as a cellular phone, a digital camera, a video camera and a notebook personal computer, or a mobile communication apparatus. Hence, the demand for such a battery has been increasingly greater. As a large power-supply system has also been developed, such a battery is in a greater demand for a back-up power-supply apparatus or an automobile-motor power source.
A lithium-ion secondary battery is formed by: a positive electrode plate made of a lithium-containing multiple oxide; a negative electrode including a negative-electrode active material which inserts and extracts a metallic lithium, a lithium alloy or and a lithium ion; and an electrolyte. Most of lithium-ion secondary batteries currently on the market are provided as a positive-electrode active material thereof with a lithium-containing cobalt multiple oxide LixCoO2 (x is a numeric value indicating the quantity of lithium contained in an active material and varies according to the charge and discharge of a battery). On the other hand, a graphite material is used for a negative-electrode active material thereof.
A battery, particularly, a lithium-ion secondary battery formed in this way has a disadvantage as follows. If some metal gets mixed into a battery in a battery manufacturing process or the like, this mixed metal may be conductive and cause an internal short-circuit between its positive electrode and negative electrode. Besides, an internal short-circuit may be triggered off through the repetition of charge and discharge, a vibration, a change in temperature or the like. Especially, since a lithium-ion secondary battery or the like is provided with an organic electrolyte, it can generate heat rapidly upon undergoing an internal short-circuit. This may generate some gas suddenly or raise the temperature, thereby bringing a safety valve provided in a sealing plate into operation. As a result, the battery can be unusable.
In order to solve these problems, a method of providing a by-pass circuit formed by a resistor or a capacitor for a battery subjected to an internal short-circuit, taking energy inside of the battery as an electric current to the outside and absorbing the energy through a resistance loss or the like is presented (e.g., refer to Japanese Patent Laid-Open Publication No. 2002-8631).
As the power and capacity of a battery has recently been increased, the energy of a battery subjected to an internal short-circuit needs to be released and absorbed in a greater amount. However, in the above described method, in order to increase the amount of energy released and absorbed from the battery, the resistance value of an energy absorber has to be made smaller to thereby raise the internal-energy release amount from this short-circuit battery. But there is a limit to such a reduction in the resistance value of the energy absorber. In short, this method is useless in coping with the recent larger capacity and power of a battery. The internal energy cannot be released enough, and thus, Joule heat is generated in a greater quantity from an electric current flowing through the part of an internal short-circuit. This causes sharp rises in the temperature and internal pressure of a battery, thus deteriorating its safety.